1. Field of the Invention
The present invention relates to a developing device, and more particularly to a developing device for rendering an electrostatic latent image visible with a one component magnetic developer.
2. Description of the Prior Art
The conventional dry developing processes for developing an electrostatic pattern are classified, with respect to the developer composition, into two-component development and one-component development. The former employs a developer composed of a mixture of carrier particles such as iron powder or glass beads and toner particles for developing the electrostatic pattern, and is associated with the drawbacks of the fluctuation of developing density resulting from the change in the mixing ratio of the carrier particles and the toner particles, and the deterioration of image quality resulting from the deterioration of the carrier particles.
On the other hand, the latter one-component development is free from the above-mentioned drawbacks due to the absence of the carrier particles, and is therefore expected to be used more widely in the future. The known one-component developer consists of toner particles incorporating magnetic powder therein for the purpose of causing frictional charge by relative movement of toner particles or transporting the toner particles to a development area facing the electrostatic image to be developed.
However the content of said magnetic powder in the toner particles is inevitably limited as the toner particles have to be bindable either by heat or by pressure to the transfer sheet in order to fix the toner image. In practice said magnetic powder constitutes 10 to 60 wt.% of the toner particles, but the volume content of said magnetic powder in the toner particles is only 20% or less because of the difference in the specific gravity of the magnetic powder and other resin components. Due to such low volume content of the magnetic powder, the toner particles show a behavior in a magnetic field different from that of the magnetic powder itself, and it becomes difficult to obtain a long low-density magnetic brush at the position of the magnetic pole. For this reason the toner layer formed on the toner supporting member, when limited to a thickness of several millimeters, tends to form an uneven distribution.
Said unevenness in the toner layer on the support member tends to be reproduced in the developed image, and the dense toner layer present on the support member except at the position of the magnetic pole may cause coagulation of toner particles or damage to the photoconductor holding the electrostatic image when said layer is pressed against the surface of said photoconductor if said layer has fluctuations in thickness. For this reason it is essential, in the development with a one-component magnetic toner, to form a thin uniform layer of toner particles on the toner supporting member.
In general, in case a thickness defining member is positioned close to the surface of the supporting member to form a slit therewith for defining the thickness of a powder layer on said supporting member when it is displaced with respect to said thickness defining member, the actually obtained thickness defined by said slit becomes slightly larger than the gap of said slit.
Because of this fact, in the development with one-component magnetic toner, it has been required, in the use of a conventional non-magnetic thickness defining member for forming a thin toner layer, to position said member very close to the toner supporting member, thereby requiring an elevated mechanical precision. Also the slit may be clogged with toner particles coagulated by various causes to hinder the tone layer formation in such clogged portion.
The use of a levelling roller for defining the thickness of one-component toner is disclosed for example in the U.S. Pat. No. 4,100,884. Also the use of an ordinary non-magnetic doctor blade for levelling the one-component magnetic toner supported on a sleeve and transported magnetically thereon by a magnet roll rotated in said sleeve is disclosed in the U.S. Pat. No. 4,014,291. However such known methods are limited to the non-magnetic levelling or doctoring means, and do not cover the use of magnetic doctoring means, principally because of the following reason.
In the conventional magnetic brush development, two-component developer or one-component developer is applied to a non-magnetic cylinder embracing a magnet. A non-magnetic doctor blade, or the like, is disposed in spaced relation with the cylinder surface to level the developer to a desired height above the cylinder surface. A non-magnetic blade is commonly used since it can provide the magnetic brush with a uniform height.
If the non-magnetic blade of such conventional device is merely replaced with a magnetic one, the magnetic field between the blade and the magnet behind the cylinder causes the brush to extend toward the blade. The extended brush will be torn by the rotation of the cylinder, thus resulting in an uneven height of the brush and leading to an undesirable development. On the other hand, in the case where what is desired is a much thinner developer layer than in the conventional development in which the magnetic brush thickness is usually in excess of 5 mm, a magnetic doctoring member is found to be effective for reducing the developer layer to a thickness for example of 0.03-0.3 mm, wherein said thin layer is maintained in facing relationship to the image bearing member with a clearance for example of 0.05-0.5 mm thereto. In such a thin layer, the eventual unevenness in the thickness caused by the use of a magnetic doctoring member is negligible because the layer thickness itself is already very small.
A method for forming, in a magnetic field, a thin and uniform layer of one-component toner on the surface of a cylinder is disclosed in the present assignee's co-pending U.S. patent application Ser. No. 938,494, filed Aug. 31, 1978 now abandoned in favor of application Ser. No. 267,771, which continuation has issued as U.S. Pat. No. 4,386,577 and a division of U.S. Pat. No. 4,386,577 has issued as U.S. Pat. No. 4,387,664.